1. Field of the Invention
The present invention relates to a handover in a broadband wireless communication system. More particularly, the present invention relates to a method and an apparatus for reducing a handover delay in a broadband wireless communication system.
2. Description of the Related Art
A broadband wireless communication system, such as a system based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard, performs a handover in order to support mobility of a terminal. A conventional handover procedure in the broadband wireless communication system is illustrated in FIG. 1. In FIG. 1, it is assumed that a terminal is connected with a Serving Base Station (SBS), and that the terminal receives a broadcast message from the SBS to perform a ranging or scanning procedure on Target Base Stations (TBSs).
Referring to FIG. 1, a terminal 101 transmits an MOB_MSHO-REQ message to the SBS 103 that the terminal 101 is in communication with in order to initiate a handover in step 111. The MOB_MSHO-REQ message includes information about TBS1 105 and TBS2 107 to which the terminal 101 is to establish communications with by performing a handover.
After receiving the MOB_MSHO-REQ message, the SBS 103, in steps 113 and 115, transmits an HO-Request message to TBS1 105 and TBS2 107 to which the terminal 101 is to handover. TBS1 105 and TBS2 107 each determine whether to allow a handover of the terminal 101, and each respond using an HO-Response message in steps 117 and 119. The SBS 103, in step 121, transmits an MOB_BSHO-RSP message to the terminal 101 in response to handover responses of TBS1 105 and TBS2 107. The MOB_BSHO-RSP message includes lists of TBSs with which the handover can be performed, and Cipher-based Message Authentication Code (CMAC) information for confirming integrity of the MOB_BSHO-RSP message. In addition, the SBS 103, in steps 123 and 125, transmits an HO-ACK associated with the HO-Response message to each of TBS1 105 and TBS2 107.
After terminal 101 receives the MOB_BSHO-RSP message, it informs the SBS 103 of the TBS with which the handover is to be performed using a MOB_HO-IND message in step 127, and performs the handover. The MOB_HO-IND message includes CMAC information for confirming integrity of the MOB_HO-IND message.
After SBS 103 receives the MOB_HO-IND message, it transmits an HO-confirm message informing of the handover of the terminal 101 to the TBS2 107 in step 129, transfers context information of the terminal 101 in step 131, and responds using an HO-ACK message in step 133. The context information of the terminal 101 may be a parameter group of the terminal. After TBS2 107 receives information about the terminal 101 from the SBS 103, the TBS2 107 performs a bearer path registration procedure in order to accurately transmit data in step 135.
Meanwhile, the terminal performs handover ranging on the TBS2 107 in step 137. In step 137, a ranging code to be transmitted is randomly selected from a handover ranging code group, and transmitted. After TBS2 107 receives the ranging code, it transmits, to the terminal 101, a physical channel correction value, etc. by transmitting an RNG-RSP message to the terminal 101, and assigns a resource through a UL-MAP to allow the terminal 101 to transmit an RNG-REQ message in step 139. The terminal 101 transmits an RNG-REQ message using the assigned resource in step 141. After the TBS2 107 has received the RNG-REQ message, it responds using an RNG-RSP message in step 143. Thereafter, normal data transmission is performed between the terminal 101 and the TBS2 107 in step 145.
However, a conventional handover procedure has a limitation that a handover delay may be generated due to code collisions between terminals that are simultaneously attempting a handover, and an additional procedure, such as those described above in steps 137 to 143, after a handover initiates.